This application pertains to computerized axial tomographic scanners and more particularly to traverse and rotate type scanners. Traverse and rotate type computerized axial tomographic scanners are well-known in the art, see for example U.S. Pat. No. 3,919,552. Also well-known in the art are the electronics and mathematics for translating the radiation attenuation through the object scanned into a visual representation of the cross section of the object scanned. See for example "The Fourier Reconstruction of a Head Section", Shepp and Logan, IEEE Transaction on Nuclear Science, June, 1974.
In a traditional traverse and rotate system, a beam of radiation and a receiving detector were scanned linearly at a constant speed across a patient in a scan circle during which time attenuation data was taken. After scanning the patient, the radiation source and detector were slowed to a stop in an area outside of the scan circle. The carriage carrying the source and detector were subsequently rotated a few degrees, and accelerated to the desired linear scan speed before reaching the scan circle. A further set of attenuation data was taken as the radiation source/detector traversed the patient in a generally opposite direction but shifted by the few degrees the carriage was rotated. This traverse, then rotate procedure was often repeated on the order of 15 to 180 times to obtain one set of data.
In the past it was commonly believed that the carriage supporting the radiation source and detector system must be traversed at a constant speed. It was believed that nonconstant traverse speeds would produce irregularities in the transmitted attenuation data. As a result only drives for the carriage were used which produced constant speeds when traversing the scan circle. Constant speed drives of the type required are complex and expensive. Because the scan velocity was constant through the scan circle all acceleration and deceleration had to occur outside of the area. The time and space required for realistic changes in speed added to the size and cost of the machine and increased the scan times.
In order to minimize the amount of human organ movement during scans, faster scan times were desired. As a result shorter acceleration and deceleration times were necessary. However, the more sudden were the changes in speed, the greater were the undesirable forces on the scanner parts. Additional wear, vibration and maintenance occured as a partial result of increased forces due to shorter acceleration and deceleration times.
To achieve the linear speed across the patient with rapid slowing and acceleration and deceleration at the extremes of the traverse, nonlinear gear boxes were often used. Ball screw drives were also used to drive the source and detector carriage through the rapidly changing speeds. Both these mechanical gearings were sometimes a source of undesirable vibration in the system.